1. Field of the Invention
The present invention relates generally to a communication apparatus which communicates data in a multiplex transmission mode between transmission and reception units having a feature in coded communication data.
2. Description of the Background Art
A previously proposed multiplex data communication apparatus includes a capacitor coupling by means of which two communication signal lines are connected with a receiving comparator to provide a plurality of differential receptions of a transmission signal to form a digital reception signal. A wired OR output from the reception comparator provides a digital transmission signal. In this case, as a noise trouble measure of the communication lines, each of comparators receiving the communication signals is coupled to the communication signal lines by means of an AC coupling.
In addition, use has been made of a PWM (Pulse Width Modulation) coding method for the communication data.
SOM (Start Of Message) which is a signal segment of the communication signals representing a start of a communication signal is expressed by a combination of "H", "H", "H", "H", "L" and "L" levels for 6 T time period, a bit "1" of the communication data is expressed by a combination of "H", "H", and "L" levels for 3 T time period, and a bit "0" of the communication data is expressed by a combination of "H", "L" and "L" for 3 T time period.
A reception technique is such that a change in a rising edge from "L" to "H" levels is detected for each pulse of the communication signals, a reception synchronization is achieved from the detected rising edge, when the detected rising edge occurs after 4 T time period of the communication signals, the signal segment represents the SOM, when the falling edge occurs after 1 T time period of the communication signals, the bit "0" is determined after 1 T time of the communication signals, when the falling edge occurs after 2 T time period, "1" is determined.
However, as a practical matter, an interline capacitance between the two communication signal lines and stray capacitance such as a unit capacitance cause waveform distortions in the pulses of the communication signals.
Furthermore, if the AC coupling is made for each reception comparator connected to the signal lines, a resistor voltage divided ratio of resistors connected between input ends of the reception comparator and coupling capacitors and a DC component of the communication signals cause a differential offset of the reception comparator to be determined. Such signal distortions and differential offset quantity as described above cause deviation in the waveforms between the transmission signal on the communication signal lines and reception signal on the reception comparator.
From a viewpoint of an electromagnetic wave interference measure, it is more effective to distort the communication signal to a larger degree. However, as the distortion of the waveform of the communication signals becomes larger, a variation in the pulsewidth become larger due to a deviation of the differential offset in the reception comparator. Since the DC component is different between the waveform indicating "0" of the communication signal and that indicating "1" of the communication signal, the DC component being an average of each waveform, the whole DC component of the communication signals is varied according to values of the communication data so that an increase and/or decrease in the pulsewidth become significant.
In addition, when either a single communication signal line is used or the reception comparator is connected in a DC coupling form to the signal communication lines, an environment around the signal communication line(s) such as GND offset between the units and a magnitude in a signal amplitude provides direct causes of deviation of the differential offset. Therefore, it is more difficult to maintain the differential offset constant.
Furthermore, a cross-talk influences pulsewidth. If a shielded wire or twisted pair is used for the communication signal line(s), an inter-line capacitance between the communication signal lines becomes larger than general AV (audio/video) lines. The stray capacitance between lines provides the AC coupling for the communication signal lines.
If one of the communication lines is broken or the one communication line is disconnected at a connector which connects the communication lines and the communication unit, the communication signal on the connected communication line is coupled into the broken communication line via the stray capacitance, thereby influencing the pulsewidth.
With the increase or decrease in the pulsewidth taken into consideration, the decoding of the PWM code is unavoidably required to provide some margin on the pulsewidth. In this case, even if the bit "1" is output to the transmission signal and the pulsewidth of the received signal becomes as short as 1/2 T time period or less, it may erroneously be determined as the signal of bit "0".
Therefore, an allowance value of an increase or decrease of pulsewidth needs to be below approximately 1/2 T (1/6 bit) time period.
In the way described below, in the previously proposed communication apparatus, since the change in the pulsewidth in the received communication signal occurs due to the distortion in the signals input to the comparator, the variation in the DC component of the communication signals, or breakage in the one communication signal line, the reception circuit or physical layer needs to be limitedly set so that the pulsewidths of the communication signals do not exceed a predetermined allowable range.